The present disclosure relates to a carbon commutator of an electric motor and a method for manufacturing the same.
There exists an in-tank fuel pump that is mounted onto a vehicle in which the fuel pump in itself is soaked (immersed) in the fuel. An inside of a housing that comprises the fuel pump serves as a flow channel of the fuel (such as gasoline). In this structure, each member that is incorporated in the fuel pump has to be made of a material with excellent fuel resistance in order to prevent a deterioration in functions due to erosion, etc., by the fuel.
As alternative fuels, alcohol (methanol or ethanol, etc.) or a mixed fuel that contains alcohol has been used in light of environmental protection. A conventional electric motor that is incorporated in a gasoline fuel pump uses a commutator segment of a commutator, with the commutator being made of copper. When alcohol fuel is directly applied to such gasoline fuel pump, copper portions of the commutator segment may be eroded by alcohol components of the alcohol fuel.
In order to prevent such an erosion by the alcohol fuel, a proposed commutator segment of a commutator comprises portions that are made of carbon (carbon base material), wherein the carbon portions are brought into sliding contact with a brush. A metal layer is formed on a side surface (a surface of a carbon base material) opposite to the brush sliding contact portions. On the metal layer, integrally formed (electrically joined) is a copper-made conductive terminal member (riser segment) (see U.S. Pat. No. 5,175,463, for example).
Surfaces of the carbon base material are so poor in wettability that most metal parts are hardly joined to the carbon base surfaces. In order to join a conductive terminal member to a carbon base material surface, for example, a metal layer has to be formed between the conductive terminal member and the carbon base material surface. In U.S. Pat. No. 5,175,463, a surface of the carbon base material is plated with nickel, etc. A conductive terminal member is joined to the plated surface by soldering, for example.
However, the metal layer that is formed by plating is easily peeled off. Although the metal layer and the conductive terminal member may integrally be joined to each other, the plated metal layer may be peeled off from the carbon base material surface, along with the conductive terminal member. The strength of such commutator may thus not be sufficiently secured. Durability of the commutator may also be insufficient.
There exists a proposed carbon base material to which a conductive terminal member is joined in advance by integrally sintering: carbon powders; metal powders that are arranged in a layer form relative to the carbon powders; and a conductive terminal member that is arranged at a side of the metal powders (see Japanese Published Unexamined Patent Application No. H8-308183, for example).
Because the carbon powders, the metal powders, and the conductive terminal member are integrally sintered, however, the sintering process is complicated and difficult to carry out. Sintering temperatures have to be set to temperatures that cause no deformation of the copper-made conductive terminal member. Such sintering temperatures also have to be lower than a melting point of the metal powders. Accordingly, sintering temperatures of the carbon base material may be limited.
If there are different shrinkage factors of the sintering between the metal powders and the carbon powders, then a gap may be easily formed between those two kinds of powders, which may soon be peeled off. In order to avoid such a problem, a carbon material has to be selected whose shrinkage factor is close to that of metal powders. Accordingly, the types of carbon material may be limited. In other words, carbon properties may be limited.
In order to solve the problem, iron powders are attached to a surface of the carbon base material, and the surface of the carbon base material is sintered at temperatures not less than diffusion temperatures of the carbon and not more than a melting temperature of the iron. Thus, formed is a functionally graded material that includes an iron layer that is integrally formed on the surface of the carbon base material. After that, the iron layer is joined to a commutator segment (copper-made plate member) by soldering, for example. As a result, a proposed material is manufactured such that the commutator segment and the carbon base material are electrically connected and integrated to each other (see Japanese Published Unexamined Patent Application No. 2002-338378, for example).
The proposed material is advantageous in that a carbon layer and a copper layer can be integrated by joining the iron layer to the copper-made commutator segment to be a commutator by soldering, for example. However, some carbon materials have lower sintering temperatures. For manufacturing the functionally graded material, if such a lower-sintering-temperature carbon material is used to produce a carbon composite base material, then a diffusion reaction hardly occurs between a binder carbonized component of the carbon material and the iron powders due to lower sintering temperatures.
In addition, a joint is not stable between the carbon and the iron powders, because a solid solution promotion, which corresponds to a carburizing reaction, may be prevented from being further promoted, thereby weakening (lowering) a joint strength between the iron layer and the carbon layer. As a result, a strength of the joint may be so low that the iron layer and the carbon layer may be peeled off from the joint by hand. In order to solve the problem, a proposed iron layer is an iron alloy layer containing a metal, such as chromium (Cr) or molybdenum (Mo), with a higher affinity with the carbon layer so as to increase an affinity with the carbon layer and an anti-peel strength (see Japanese Published Unexamined Patent Application No. 2004-208398, for example).